Tick-borne bacterial pathogens of humans cause significant morbidity and mortality throughout the United States and abroad. Lyme disease (LD), caused by Borrelia burgdorferi, is the most prevalent arthropod-borne disease of humans in the United States and many other countries throughout Europe and Asia. Tick-borne relapsing fever (TBRF), caused by Borrelia hermsii, is endemic in scattered foci throughout many regions of higher elevation in the western United States. Our work has focused on three areas: 1) to improve on the serodiagnosis of LD and TBRF fever by using recombinant DNA technology to clone genes of spirochetes that express proteins that induce specific and detectable antibody responses; 2) to examine how spirochetes adapt to their tick and mammalian hosts; 3) perform genetic analysis of spirochete isolates and comparative genomics between relapsing fever and Lyme disease spirochetes. This work requires that we maintain colonies of Ixodes scapularis and Ornithodoros hermsi, the respective tick vectors of LD and TBRF spirochetes, and infect these ticks via a laboratory mouse - tick cycle. Our interests in serodiagnosis and adaptations associated with tick infection and transmission demand that we have a solid understanding of genetic and phenotypic variation within our species of interest. During the last 17 years we have acquired new, low passaged isolates of B. hermsii and Borrelia turicatae. We now have 31 isolates of B. hermsii, which have come from localities throughout a major part of the known distribution where relapsing fever has been associated with the occurrence of the tick vector, Ornithodoros hermsi. DNA analysis of all the isolates is done and includes complete sequences of 5 genes that has identified two distinct clusters, which we have designated Genomic Group I and Genomic Group II. Our collection of new isolates of relapsing fever spirochetes has been critical for our expansion of our genomic project in which we are sequencing the genomes of B. hermsii and B. turicatae. We have completed the DNA sequencing of the linear chromosome of Borrelia hermsii and Borrelia turicatae. The gene organization of the chromosome of both species is nearly identical, with 816 open reading frames encoded on linear molecules of approximately 916,000 base pairs. Both relapsing fever species have 22 genes on their chromosome that are absent in the entire genome of the Lyme disease spirochete, B. burgdorferi. Pathways for glycerol-3-phosphate metabolism, purine salvage and additional genes for recombination and DNA repair are unique to the relapsing fever spirochetes. We have also identified a putative reticulocyte-binding protein that may be involved with B. hermsii adhering to erythrocytes. Our goal is to identify unique pathways and mechanisms that control the difference in pathogenicity of the relapsing fever spirochetes, which allows them to achieve high cell densities in the host's blood, which is an essential adaptation for being acquired by fast-feeding ticks.